1. Field of the Invention
This invention relates to a method for enhancing the cooking step in a tall oil preparation process comprising a neutralisation step, in which soap is neutralised to form soap oil, and a cooking step, in which soap oil is cooked with sulphuric acid to form tall oil. The invention also relates to a method for preparing tall oil comprising the steps above.
2. Description of the Related Art
In the alkaline digestion of softwood or hardwood, especially in the sulphate method, xe2x80x9cblack liquorxe2x80x9d is formed, which contains crude tall oil soap, called crude soap below. This crude soap can be decomposed by means of sulphuric acid to form crude tall oil.
U.S. Pat. No. 3,901,869 discloses a process for preparing tall oil based on carbon dioxide neutralisation. In the first step of this known process, crude soap is neutralised with carbon dioxide and after this step water is separated and finally tall oil cook is performed by means of sulphuric acid. When crude tall oil is treated with carbon dioxide, carbon dioxide dissolves in the soap/water emulsion and the carbon oxide formed neutralises soap and the lye accompanying it. The pH of the soap/water mixture can be decreased to the range 7.5-8.2 by neutralisation. This known method involves the drawback of poor water separation. Poor water separation results in excess consumption of sulphuric acid in the subsequent cook, and the water surplus expands the process cycles unnecessarily. A surplus of water containing CO2 also entails effervescence in the sulphuric acid cook, because bicarbonate decomposes when cooked.
WO Patent Application 94/11571 discloses a method for preparing tall oil by first performing preoxidation with a sodium bisulphate solution, followed by a tall oil cook with sulphuric acid.
FI Patent Application 940969 describes a method, in which carbon dioxide neutralisation is followed by a second neutralisation by means of an H2SO4 addition. With this procedure, soap separation in the water separation step was improved compared to a method where no intermediate neutralisation with sulphuric acid was performed. After this intermediate neutralisation, a tall oil cook is performed by means of sulphuric acid.
FI Patent Application 952176 discloses a similar method, in which the sulphuric acid used for intermediate neutralisation is replaced with an acidly reacting substance containing bisulphite.
The purpose of the invention is to further improve the tall oil preparation process, particularly the cooking step of the process.
The chief characteristics of the invention are defined in the accompanying claims.
Thus, the invention provides a method for enhancing the cooking step in a tall oil preparation process comprising:
a neutralisation step, in which soap is neutralised to form soap oil, and a cooking step, in which soap oil is cooked with sulphuric acid to form tall oil, the method being characterised by the fact that the soap oil obtained in the neutralisation step is subjected to an intermediate treatment before the cooking step, the intermediate treatment comprising heating the soap oil in order to release gases dissolved in and bound to the soap oil.
In addition, the invention provides a method for preparing tall oil comprising:
a neutralisation step, in which soap is neutralised to form soap oil, and a cooking step, in which soap oil is cooked with sulphuric acid to form tall oil, the method being characterised by the fact that it also comprises an intermediate treatment, in which the soap oil obtained in the neutralisation step is heated before the cooking step in order to release gases dissolved in and bound to the soap oil, thereby facilitating the cook and enhancing the phase separation following the cook.
Owing to the intermediate treatment of the invention, soap oil is easier to cook, i.e. the final cook requires less sulphuric acid and/or shorter cooking periods, and the phase separation following the cook is rapid.
The intermediate treatment mentioned above is a separate step between the neutralisation step and the cooking step, and no chemicals are added in this step. The intermediate treatment can be performed in a separate reactor, but it is also possible to perform the intermediate treatment and the cooking step consecutively in the same reactor.
The intermediate treatment is preferably performed at a temperature in the approximate range from 60 to 100xc2x0 C., and most preferably at a temperature in the approximate range from 90 to 98xc2x0 C. The intermediate treatment duration is preferably 5 minutes or more, and most preferably approximately 5 to 30 minutes.
The neutralisation step in accordance with the invention may comprise neutralisation of soap with an acidly reacting substance, which is preferably carbon dioxide, bisulphite, sulphur dioxide or carboxylic acid, or a mixture of these.
The neutralisation step of the invention may also comprise soap neutralisation first with carbon dioxide and then with a second acidly reacting substance, which is preferably bisulphite, sulphur dioxide, sulphuric acid, hydrochloric acid, nitric acid or carboxylic acid, or a combination of these.
The carboxylic acid mentioned above may be for instance formic acid.
The bisulphite used for neutralisation may consist of a residual solution produced in the additional scrubbing of sulphurous flue gases from a pulp mill, the solution containing sodium sulphite (Na2SO3) besides sodium bisulphite (NaHSO3).
The neutralisation step preferably comprises neutralisation with carbon dioxide. CO2 neutralisation can be performed under normal pressure or over-pressure, typically under a pressure of about 8 bar. The carbon dioxide used is preferably a carbon dioxide gas.
The intermediate treatment is preferably conducted under atmospheric pressure, however, higher or lower pressures can also be applied.
In this invention, the term xe2x80x9csoap oilxe2x80x9d denotes crude soap pre-neutralised with carbon dioxide or some other acid chemical.